PROJECT ABSTRACT/SUMMARY Law enforcement officers (LEOs) are exposed to significant stressors, elevating their risk for aggression and excessive use of force. Such dysfunctional stress reactivity can lead to devastating consequences for their community, including unjustified shootings, severe beatings, and fatal chokings. It can also lead to serious consequences for the LEOs, including elevated incidence of post-traumatic stress disorder, burnout, alcohol misuse, depression, and suicide. A recent meta-analysis of LEO stress reduction programs found little evidence to demonstrate that such approaches are effective, highlighting the urgent need for preventive interventions targeting the stress inherent to policing. Mindfulness training is a promising approach with high- stress populations that has been shown effective in reducing stress and increasing resilience. Our recent pilot feasibility study (R21AT008854) established initial feasibility, acceptability, and adherence to procedures in a single-site RCT assessing Mindfulness-Based Resilience Training (MBRT), a preventive intervention designed to improve LEO mental health and resilience, and reduce aggression and excessive use of force. The R21 data suggest physiological mechanisms and potential clinical benefit in a sample of LEOs. Relative to waitlist control, MBRT participants showed improvements in stress reactivity, aggression, burnout, occupational stress, sleep disturbance, and psychological flexibility. Implemented at two sites, the proposed study is designed to establish optimal protocols and procedures for a future full-scale, multi-site trial assessing effects of MBRT versus an attention control (Stress Management Education) and a no-intervention control, on physiological, behavioral, and psychological outcomes. To prepare for this future clinical trial, this study will: a) enhance efficiency of recruitment, engagement and retention; b) optimize lab, assessment, and data management procedures; c) optimize intervention training and ensure fidelity to intervention protocols; and d) assess participant experience and optimize outcome measures across sites. The long-term objective of this line of research is to develop an intervention that will reduce violence and increase resilience among LEOs, as well as yield significant benefits for communities and residents they serve.